1. Field of the Invention
The present invention relates to industrial type non-contact detection systems for ascertaining the state of an object in a machine system. More specifically, the present invention relates to a laser detection system for ascertaining the state of rotating objects or objects in some other continuous repetitive motion.
2. Description of the Prior Art
The development of machinery for automated manufacturing has produced many benefits to mankind. More specifically, today, the manufacture of most consumer and industrial electronic instruments and appliances is largely automated. During the manufacture of electronics boards, holes must be precisely drilled in a circuit board for interconnecting circuits on the board and for fastening the boards to a chassis. For this purpose, high speed, computerized drilling machines rapidly position the board, hold it in place, and drill the necessary holes.
However, problems arise when a drill bit breaks during the drilling operation and the broken bit goes undetected. If the broken bit is not detected, the drilling machine will attempt to continue drilling holes with serious consequences On the one hand, if the drill breakage occurs only a small distance from the tip of the bit, the drilling machine will continue to drill with the damaged drill bit, thus destroying the board or, at best, producing unacceptably poor quality holes and, in either case, this may require the defective circuit board to be discarded. On the other hand, if the drill breakage occurs at a greater distance from the drill tip, the resultant circuit board will lack holes which were to be created in the drilling process subsequent to the bit being damaged A defective board lacking one or more holes may be recognized during a post-inspection process and the missing holes may be drilled However, this corrective procedure causes a significant reduction in productivity
While non-contact systems for detecting broken bits are presently available, conventional non-contact systems are typically highly affected by environmental conditions and are unable to be conveniently located on the drilling system Presently known non-contact detection systems include those using light emitting diode (LED) or microwave technologies, both of which are highly susceptible to environmental conditions. These non-contact detection systems do not utilize the benefits of the continuous repetitive movement of the object to eliminate error, interference and the effects of harsh changing environmental conditions.
Also, many available detectors are required to be located very close to the object and at an angle normal to the objects motion. Generally, this is a poor location and subjects the detection device to difficult operating conditions. In a drilling machine operation, the traditional light detection systems must be placed in a ballistic path of the debris generated by the drill increasing interferences and device errors.
LED detection systems utilize the principle of light occlusion thus requiring the object being detected to be directly in the path of the light. On drilling systems, this necessitates the full retraction of the drill bit into a pressure foot assembly This full retraction after each hole is drilled greatly decreases the efficiency of the drilling machine. Also, such a system must detect small fractional signal changes, particularly for drills with smaller diameters. However, the resulting LED, or occlusion, system has a relatively low signal-to-noise ratio making such signal ranges difficult to detect.
Non-contact microwave detection systems operate on the principle that the presence of a continuous target, such as a fine drill, in or near the end of the waveguide at a predetermined location produces a change in the standing wave ratio (SWR) of the transmitted signal This change in the SWR is capable of being detected by a receiver to provide a signal that is indicative of the target condition. These microwave systems do not operate using the Doppler principle of measured frequency change; rather, only the DC component of the signal is utilized by such detection systems Hence, the frequency components of the output signal are not used.
The creation of a resonant microwave cavity and the detection of objects entering the cavity by a change in the SWR is affected by nearly any foreign object entering the resonant cavity Thus, such a system is prone to interference from airborne debris. The microwave detection systems are also subject to interference from the copper shavings produced during the drilling of copper layers in printed circuit boards, known as "drill-wrap", and system to system variations.
In addition, all elements in or surrounding the microwave cavity must remain constant The components making up the cavity cannot be moved without making changes or adjustment to the microwave detection device. The microwave detection system is not easily adaptable from system to system and adjustments or variations may be needed depending on the objects in and around the cavity created by each host system.
The microwave detection systems produce small signal changes from the object-present state to the object-not-present state which are analyzed by the system's comparator. Using a small signal change greatly increases the possibility of error by the microwave system
In addition to the aforesaid disadvantages of the presently known drill bit detections systems, these systems do not detect when a drill bit is clogged. Because of the environmental affects on LED detection systems, and the resulting low signal-to-noise ratio, it is difficult to obtain a signal that is measurably indicative of the varying reflectivity of a spinning drill bit It will also be appreciated that the present microwave systems utilize only the DC component of the reflected signal and, therefore, do not take into account the frequency of the reflected signal.
Many of the disadvantages of the LED and microwave systems have been overcome by the advent of a laser detection system which detects fluctuating light reflected from a drill bit Such a laser detection system is presently included in the Concept 1 & 4, MVI and MVII drilling machines manufactured by Excellon Automation of Torrance, Calif. Nevertheless, while the present laser detection system is advantageous for detecting the presence or absence of a drill bit, it does not detect whether a drill bit is clogged.
A drill bit may be become clogged, for example, when "green" printed circuit boards are being drilled. Circuit boards are typically formed from an epoxy resin material which must be cured. Therefore, a green, or incompletely cured, board contains tacky material that may stick to the drill bit during drilling. A drill bit may also become clogged when the feed rate of the drill bit is too low. That is, if the axial motion of the bit through the work is too slow, the drill bit will heat up, heating surrounding board material by conduction, and again providing a means for tacky material to be deposited in the flutes of the drill bit. A clogged bit has a reduced reflective surface area that causes the present laser detection system to sometimes indicate that the drill bit is broken. Regardless of how material is deposited in the flutes, it is often important to distinguish between a clogged drill bit and a broken drill bit.
In general, when a drill bit breaks, the drilling of a circuit board must stop until the drill bit is replaced on the other hand, when a drill bit is clogged, the drilling process need not necessarily stop. Oftentimes, the drill bit will clog but it is unnecessary to clean the drill bit until many boards have been drilled. Therefore, if the bit detection system cannot distinguish between whether a bit is broken or just clogged, drilling time is lost due to unnecessary delays in checking clogged bits It is also vital to provide the user of the drilling machine with the option of either stopping the machine or continuing the drilling.
Consequently, a need exists for a drill bit detection system which can accurately detect the state of an unbroken drill bit. Of present significance, the drill bit should be determined to be clean, clogged or broken. In addition, if the drill bit is clogged, the operator of the drilling machine should be given the option to stop or continue drilling.